Short pulse laser systems generate optical pulses having sub-microsecond pulse width and sub-millisecond temporal spacing. As used herein, a short pulse of light is an electromagnetic pulse whose time duration is less than a microsecond (10−6 seconds). Short pulse laser systems include ultrashort pulse laser systems (or conversely ultrafast laser systems) such as nanosecond, picosecond and femtosecond laser systems and other laser systems that can produce amplified optical pulses with sub-microsecond pulse width and sub-millisecond temporal resolution.
In conventional short pulse laser systems, high energy pulses are produced at a constant frequency and constant energy. Changing the time between pulses in such lasers can produce significant differences in the energy of each pulse with the potential for damage to the laser or the object to which the pulses are applied. There is a need for short pulsed lasers that can be triggered at any arbitrary time to provide a short duration pulse while controlling the energy of each triggered optical pulse. Such a laser would be advantageous for micromachining and potentially in other fields (e.g., ophthalmology, biomedical imaging, ultrafast spectroscopy, ultra-high-speed optical networks, reaction triggering, femtochemistry, etc.).
Taking conventional ultrafast laser micromachining as just one example, an optical beam is moved on a workpiece to apply the beam in a specific pattern. The beam can be moved, the workpiece can be moved, or both can be moved to trace the pattern. In order to maximize the processing speed, this movement should be as fast as possible; however, the speed is limited by the requirements on the accuracy of the movement. The movement can be very fast for straight lines, but may need to be very slow for small or complex features. The transitions between fast and slow movement occur dynamically, depending on the complexity of the pattern being traced.
When micromachining with ultrafast lasers, the optical beam being applied to the workpiece comprises nanosecond to femtosecond duration optical pulses with a pulse repetition frequency (PRF) in the range of kHz to MHz. Ideally, the energy of each pulse is kept constant and the pulses are evenly spaced in physical location on the workpiece. The spacing of pulses on the workpiece is proportional to the scanning speed (the relative speed between the optical beam and the workpiece) and the time between optical pulses (or inversely, the pulse repetition frequency (PRF) of the optical beam). Because the scanning speed dynamically changes, the time between optical pulses must also dynamically change to keep spacing of pulses constant on the workpiece. Unfortunately, dynamically changing PRF of a conventional amplified ultrafast laser will change the energy of the output optical pulses. In FIG. 1, the solid line is the amplifier's (e.g., the amplifier of a conventional ultrafast laser) stored energy, the input pulses are represented by the filled circles, and the output pulses are represented by the hollow circles. The relative energy level of a pulse is represented by the diameter of its solid or hollow circle.
FIG. 1 graphs the stored energy of an amplifier of a conventional ultrafast laser (such as an ultrafast laser according to U.S. Pat. No. 7,386,019 “Light pulse generating apparatus and method” published Jun. 10, 2008 incorporated herein by reference in its entirety). The solid line represents the amplifier's stored energy over time. The solid circles on the graph represent input pulses entering the amplifier where the size of the solid circle represents the energy of the input pulse. The outlined circles represent output pulses, concentric with their corresponding input pulse, and the size of the hollow circle represents the energy of the output pulse. In the example of FIG. 1, the energy of all input pulses remains the same. Conventionally, and as illustrated for the first three input pulses, the temporal spacing is fixed based on a pulse repetition frequency of a source (e.g., a short pulse laser source, etc.) providing the pulses to the amplifier. For the first three pulses, there is fixed temporal spacing, the stored energy is in dynamic equilibrium, returning to a target level at the same time the next pulse arrives, and the output pulses accordingly have a constant energy level. When the temporal spacing between pulses changes, the amplifier's dynamic equilibrium is broken. Where the time between pulses increases, the amplifier overshoots the target and the energy of the next pulse can be so great that the pulse can damage the laser, the workpiece or other things. When the time between pulses decreases, the amplifier cannot replenish its stored energy to the target level and the energy of the next pulse may be insufficient to perform its task. If the timing of even one pulse is sufficiently off, the amplifier can take a significant amount of time to re-establish its equilibrium. These problems compound as more pulses deviate from a constant temporal spacing, or a constant pulse repetition frequency. Further problems occur if the energy of each input pulse is not held constant.
The lack of arbitrarily timed, or arbitrarily triggered pulsed lasers that keep pulse energy fluctuations sufficiently small, results in compromising on either throughput, quality or system complexity. In many of today's ultrafast laser applications and market areas, this is a very significant limitation. Accordingly, it would be advantageous to have a pulsed laser that can be triggered at arbitrary times, with high temporal resolution, to provide a short duration optical pulse in association with each trigger while being able to control the energy of all the optical pulses.